There has recently been an increasing demand for alkaline secondary batteries with high power and longevity as batteries for power tools or electric automobiles. There has also been a desire for improvement in battery characteristics such as improvement in charge efficiency at a high temperature and discharge characteristic at a low temperature as well as reduction in self-discharge.
Conventionally, a sintered-type nickel positive electrode for an alkaline secondary battery has been produced in the following manner:
First, slurry obtained by mixing a nickel powder, a thickener and water is applied onto a core member, and is sintered in a reducing atmosphere to produce a sintered nickel substrate. This sintered substrate is soaked in a nitric acid aqueous solution containing nickel nitrate dissolved therein (hereinafter referred to as an acidic nickel salt solution) to impregnate nickel nitrate in the pores of the sintered substrate, and is dried and then soaked in an alkaline solution. Thereby, nickel nitrate changes to hydroxide as an active material. Repetition of this operation leads to filling of a pre-determined amount of active material in the pores of the sintered nickel substrate.
There is a problem in the step of soaking the sintered nickel substrate in the acidic nickel salt solution that the surface of the sintered nickel substrate corrodes due to the corrosive effect of the acidic nickel salt solution, which decreases the strength of the sintered nickel substrate. Herein, one of the modes in which a battery life comes to an end is an increase in battery internal resistance attributed to uneven distribution of an electrolyte caused by swelling of a positive electrode plate. The causes for the swelling of the positive electrode include a change, during overcharge of the battery, of xcex2-Ni(OH)2 which is the main component of the active material, to xcex3-NiOOH which has a low density. As the strength of the sintered nickel substrate decreases, as thus described, the swelling of the electrode plate is further accelerated, making the cycle life of the battery shorter.
As solutions to this problem, it has been proposed in Japanese Laid-Open Patent Publication No. Hei 4-75257 that cobalt oxide is produced on the surface of the sintered nickel substrate to suppress the corrosion of the substrate. In this case, the sintered nickel substrate is soaked in a cobalt salt aqueous solution and dried at a temperature of from 90 to 100xc2x0 C., and is then treated with an alkaline solution to produce cobalt hydroxide on the substrate surface. After this cobalt hydroxide is oxidized, the substrate is soaked in the acidic nickel salt solution. Namely, the oxidation of cobalt hydroxide is conducted for the purpose of suppressing the corrosion of the substrate by the acidic nickel salt solution. However, there is a problem in this proposal that, while the corrosion of the sintered nickel substrate upon impregnation of the active material is suppressed, the active material in the vicinity of the sintered nickel substrate is prone to change to xcex3-NiOOH in charging, undesirably deteriorating the cycle characteristic.
On the other hand, it has been proposed in Japanese Laid-Open Patent Publication No. Hei 11-102700 that coprecipitated hydroxide of cobalt and magnesium is produced in the pores of the sintered nickel substrate, and the substrate is then soaked in an acidic nickel salt solution so that an active material is filled in the substrate. It can be considered that this proposal is to enhance the operating voltage of the battery by producing, in soaking the substrate in the acidic nickel salt solution, a solid solution layer on a solid phase interface between coprecipitated hydroxide of cobalt and magnesium and the active material comprising nickel hydroxide. A problem lying in this proposal is that the production of the solid solution layer allows the operating voltage to increase, but is undesirably prone to promote the decomposition of the active material in the charged state, resulting in deterioration of the self-discharge characteristic and reduction in charging efficiency in a high-temperature atmosphere.
It is an object of the present invention to attain an increase in longevity of an alkaline secondary battery without sacrificing other characteristics thereof, such as a self-discharge characteristic.
The present invention relates to a method for producing a sintered-type nickel positive electrode for an alkaline secondary battery, comprising:
a step (1) of soaking a sintered nickel substrate in an acidic solution containing cobalt ions and at least one metal ions selected from the group consisting of magnesium ions, iron ions and manganese ions, and drying thus soaked substrate to prepare a metal salt carried-substrate A;
a step (2) of soaking the substrate A in an alkaline solution to deposit cobalt hydroxide and at least one selected from the group consisting of magnesium hydroxide, iron hydroxide and manganese hydroxide deposit in pores of and on the surface of the substrate, to prepare a hydroxide carried-substrate B;
a step (3) of oxidizing the cobalt hydroxide to produce cobalt oxide having a mean cobalt valence of over 2, to prepare an oxide carried-substrate C; and
a step (4) of filling an active material comprising nickel hydroxide in pores of the substrate C by repeating a series of steps of soaking the substrate C in a solution containing nickel nitrate dissolved therein, drying thus soaked substrate C, and then soaking thus dried substrate C in an alkaline solution, to prepare an active material carried-substrate D.
It is preferable that the step (3) comprises steps of providing the substrate B with an alkali and exposing the alkali provided-substrate B to an air atmosphere at a temperature of not lower than 100xc2x0 C. and not higher than 150xc2x0 C. to oxidize the cobalt hydroxide to produce cobalt oxide.
Alternatively, it is preferable that the step (3) comprises a step of electrochemically oxidizing the substrate B in an alkaline solution to oxidize the cobalt hydroxide to produce cobalt oxide.
It is preferable that the alkaline solution used in the step (2) is an aqueous solution containing sodium hydroxide dissolved therein.
It is preferable that in the acidic solution used in the step (1), the concentration of the total amount of the cobalt ions and at least one metal ions selected from the group consisting of magnesium ions, iron ions and manganese ions is not less than 1.2 mol/liter and not more than 2.0 mol/liter, and the proportion of the cobalt ions in the total amount is not less than 30 mol % and not more than 70 mol %.
It is preferable that the method of the present invention further comprises a step (5), subsequent to the step (4), of filling at least one additive selected from the group consisting of yttrium hydroxide, ytterbium hydroxide and calcium hydroxide in pores of the substrate D by: soaking the substrate D in an acidic solution containing at least one metal ions selected from the group consisting of yttrium ions, ytterbium ions and calcium ions; drying thus soaked substrate D; and then soaking thus dried substrate D in an alkaline solution, to prepare an additive carried-substrate E.
It is preferable that in the substrate E, the total amount of at least one selected from the group consisting of yttrium, ytterbium and calcium is not less than 0.2 mol % and not more than 2 mol % relative to the amount of nickel contained in the active material.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.